In order to satisfy the demands of various wireless services, more and more antennas are accommodated in one physically compact mobile terminal. Accordingly, the signal isolation between antennas becomes more and more insufficient, which causes severe radio frequency interferences among the collocated antennas. In fact, such coexistence interferences impact almost all modern wireless communication devices. Taking a mobile phone or a wireless router for instance, various communication systems, including 2G (GSM), 3G (UMTS), 4G (LTE), Wi-Fi, GPS and Bluetooth, coexist in a very compact volume, and operating frequency bands of which are very close to each other. Thus, the mutual coupling between antennas is severe, which leads to a low radiation efficiency of the antennas. Even worse, when mutual coupling is strong, the power will be coupled from one antenna to other antennas rather than radiating to free space, thus decreasing the signal-to-noise ratio and data throughput. These effects will eventually deteriorate the performance of the collocated systems which operate in adjacent frequency bands.
Using multiple antennas is also an effective way to overcome fading effect and to increase the spectrum efficiency. There are two main applications: Spatial (or polarization) diversity is used to enhance the reliability of the system with respect to various of fading; and Spatial multiplexing is used to provide additional data capacity by utilizing the different uncorrelated paths to carry additional data streams. The latter is referred to as Multiple Input Multiple Output (MIMO) data access scheme.
When multiple antennas are implemented, inverted F antenna (IFA), loop and monopole antennas are three popular antenna forms used in mobile terminals due to their simplicity and compactness in structure, flexibility in design and multiple-band options.
Nevertheless, no matter what antenna form is used, because of the compact volume of a mobile terminal and co-existence of many antennas, severe mutual couplings among the multiple antennas will inevitably decrease the Signal-Noise-Ratio (SNR) and increase the signal correlation. Additionally, a strong mutual coupling also lowers the radiation efficiency. All of these negative effects decrease the superiority of a multiple antenna system and deteriorate the system performance.
There are mainly four categories of known decoupling methods: 1) adding a neutralization line between two coupled antennas to reduce the mutual coupling; 2) destroying the ground plane between two coupled antennas to alert the current on the ground between two coupled antennas; 3) inserting parasitic elements between coupled antennas; and 4) introducing a decoupling network either shunt connected between the coupled antennas or cascade connected between coupled antenna ports and transmitter/receiver ports.
Though these approaches can help to improve the isolation between two antennas, many of them are ad-hoc to a specific antenna configuration and they all require to introduce either an interconnecting circuit or an electromagnetic structure between two antennas. This requirement either increases the whole footprint in antenna layout or inter-connects two antennas by a block of structure, all in a dimension of a large fraction of wavelength. All these approaches are difficult to implement in most of practical mobile terminals. Such situation is more challenging at low frequencies. The concurrent trends of a wireless terminal tend to have a less clearance for antennas and more collocated antennas, which severely limits the use of these existing decoupling schemes in practical applications. It is clear that having a decoupling method that has no physical connection between two coupled antennas and occupies virtually no extra space will be highly appreciated by the industry, not mentioning that if the decoupling method is simple to implement.